This invention relates to combustion systems, and more specifically, to a fluidized bed combustion system that is particularly suited for use for the purpose of effecting the combustion therewithin of a wide range of fuels of varying quality and moisture, and especially fuels such as high moisture wood waste and paper de-inking solids.
In a number of countries, forests are more than a source of lumber, pulp and paper. They are also an important component of the landscape and ecology of these countries. In addition, they constitute a major source of comparative advantage and foreign exchange for these countries as well as comprising the economic backbone of many of these countries' communities.
It can be expected that the pulp and paper industry during the 1990's will continue to spend heavily on environmental programs, and in particular on those that are designed to reduce effluent emission. In this regard, secondary effluent treatment, it is being found, is causing an increase in the amount of sludge that is being generated by many paper mills. Generally speaking, such sludges presently must either be landfilled or incinerated.
Coupled with the foregoing is the fact a major market force today in the pulp and paper industry, as reflected in the rapid growth in this market segment, is the demand for recycled paper products. Such paper recycling as well as the de-inking operations associated therewith can be expected to add to the quantity and character of the sludges that are presently being generated on-site within many paper mills.
To this end the period of rapid growth that paper recycling presently enjoys will no doubt drive the development and application of a host of technologies. Moreover, installing recycling and de-inking facilities will enable paper mills to produce products that will have a growing appeal to consumers around the world. Thus, the challenge, which the pulp and paper industry faces, is one of viewing the by-products of the de-inking process as being more than just another disposal problem.
Currently, the de-inking process being most commonly employed is flotation in which waste paper is washed and treated with NaOH. This treatment causes a swelling of the fibers, which in turn tends to loosen the ink particles and coating materials that are contained in the waste paper. Peroxides (H.sub.2 O.sub.2) and surfactants are then added to bleach or "whiten" the fibers and to disperse the ink particles. The ink particles become hydrophobic in the process and attach themselves to rising air bubbles thereby enabling them to be removed in the form of a foam. The ink particles so removed together with the coating material that is removed and along with the rejects and water collectively form a wet mass that is commonly referred to as "de-inking sludge." Present mechanical dewatering techniques can only reduce the moisture content of the de-inking sludge to between 40% and 60% due to the sponging effect of the waste fiber contained therewithin. A typical de-inking plant of 250 tons per day capacity will yield, as by-products of the paper recycling and de-inking operations, approximately 20 bone dry tons per day of paper de-inking solids from the de-inking sludge.
As attention is being focused on what to do with these paper de-inking solids, various options are being considered. For example, at least one paper mill has initiated a program to truck the paper de-watering solids to local farmers who use the paper de-inking solids as a solid conditioning additive. On the other hand, landfilling the dewatered paper de-inking solids is often found to be the least expensive method of disposal thereof. However, local regulations and economics may dictate developing other methods of disposal of the paper de-inking solids, particularly should the production of such paper de-inking solids surge over the next few years as is currently being anticipated.
One of these other methods of disposal of paper de-inking solids is considered to be incineration. In this regard, an advantage that incineration is perceived to possess is that it permits the inorganic component of the paper de-inking solids to be recovered for possible reuse.
It is long been known in the prior art to provide combustion systems that are suitable for employment for purposes of effecting the incineration of materials. More specifically, the prior art is replete with examples of various types of combustion systems that have been used heretofore to effect the incineration of a multiplicity of different kinds of materials. In this regard, in many instances discernible differences of a structural nature can be found to exist between individual ones of the aforesaid combustion systems. The existence of such differences is in turn attributable for the most part to the diverse functional requirements that are associated with the individual applications in which such combustion systems are designed to be employed. For instance, in the selection of the particular type of combustion system that is to be utilized for a specific application one of the principal factors to which consideration must be given is that of the nature of the material that is intended to be incinerated through the use of the combustion system.
Waste material is one such material wherein combustion systems have been utilized for purposes of effecting the incineration thereof. Furthermore, fluidized bed combustion systems represent one such form of combustion system that has been utilized in this regard. By way of exemplification and not limitation, one example of a prior art form of fluidized bed combustion system that has heretodate been utilized for purposes of effecting therewith the incineration of waste material is that which forms the subject matter of British Patent No. 1,299,125 entitled "Improvements in Fluidized Bed Incineration," which was published on Dec. 6, 1972. In accordance with the teachings of British Patent No. 1,299,125, a method and apparatus for effecting the incineration of combustible refuse is provided wherein a bed of hot particulate refractory material is provided in an incinerator vessel having a first opening above one region of the bed and a second opening adjacent the base of the bed at a second region spaced horizontally from the first region. The bed of hot particulate refractory material is fluidized in a non-uniform manner to cause a greater degree of agitation of the bed at the second region than at the first region thereby promoting circulation of the material of the bed in the incinerator vessel in the direction downwardly from the first region and towards the second region. Through the first opening and onto the surface of the bed of the hot particulate refractory material there is introduced a mixture of combustible and non-combustible refuse such that the combustible content of the refuse is burned in the bed and the non-combustible content of the refuse is withdrawn through the second opening.
Another example, by way of exemplification and not limitation, of a prior art form of fluidized bed combustion system that has heretodate been utilized for purposes of effecting therewith the incineration of waste material is that which forms the subject matter of U.S. Pat. No. 4,419,330 entitled "Thermal Reactor of Fluidizing Bed Type," which issued on Dec. 6, 1983. In accordance with the teachings of U.S. Pat. No. 4,419,330, there is provided an incinerator of the fluidized bed type for effecting therewith the incineration of municipal refuse. The subject fluidized bed type incinerator includes a blower that supplies fluidizing gas upwardly into the incinerator through a diffusion means disposed at the lower part of the incinerator so as to fluidize the fluidizing medium or sand above a plate means. The fluidized medium is forced to move upwardly adjacent the side walls of the incinerator by the upwardly injected gas whereby the flow of the medium is directed against inclined deflecting walls such that whirling fluidized flows are created there as well as a downwardly descending bed between the whirling flows. Due to the presence of the whirling fluidized flows and the downwardly descending bed, the municipal refuse is alleged to be satisfactorily incinerated without obstruction to fluidization even though preshredding of the municipal refuse is not performed before the municipal refuse is charged into the incinerator.
Still another example, by way of exemplification and not limitation, of a prior art form of fluidized bed combustion system that has heretodate been utilized for purposes of effecting therewith the incineration of waste material is that which forms the subject matter of U.S. Pat. No. 4,823,740 entitled "Thermal Reactor," which issued on Apr. 25, 1989. In accordance with the teachings of U.S. Pat. No. 4,823,740, there is provided a thermal reactor of the fluidized bed type for effecting the incineration therewith of municipal waste wherein the fluidizing medium is caused to produce substantially two circulating zones A and B between which there exists a descending bed. Moreover, the materials to be burnt in the descending bed are entrained therein due to the presence of the oppositely circulating zones A and B. The subject thermal reactor in addition is provided with chambers on the outermost sides of each of the circulating zones A and B whereby a part of the fluidized bed under fluidization is directed into each of these chambers such as to thereby enable thermal energy to be recovered from the heated fluidizing medium passing therethrough.
Yet another example, by way of exemplification and not limitation, of a prior art form of fluidized bed combustion system that has heretodate been utilized for purposes of effecting therewith the incineration of waste material is that which forms the subject matter of U.S. Pat. No. 4,879,958 entitled "Fluidized Bed Reactor with Two Zone Combustion," which issued on Nov. 14, 1989. In accordance with the teachings of U.S. Pat. No. 4,879,958, there is provided a fluidized bed thermal reactor wherein circulating refractory material and fuel form a pair of fluidized beds, each revolving side by side. The fluidized bed thermal reactor also includes a hollow body which serves to divide the thermal reactor into an upper combustion zone and a lower combustion zone, and wherein by selecting the gas flow through the base of the reactor and by selectively positioning the deflector surfaces of the aforementioned hollow body, the desired flow direction of refractory material and fuel can be achieved, i.e., upwardly at the center of the thermal reactor and outwardly and downwardly at the outer edges thereof.
Yet still another example, by way of exemplification and not limitation, of a prior art form of fluidized bed combustion system that has heretodate been utilized for purposes of effecting therewith the incineration of waste material is that which forms the subject matter of U.S. Pat. No. 5,138,982 entitled "Internal Circulating Fluidized Bed Type Boiler and Method of Controlling the Same," which issued on Aug. 18, 1992. In accordance with the teachings of U.S. Pat. No. 5,138,982, there is provided a circulating type fluidized bed incinerator wherein the fluidizing medium at the portion near the side wall thereof that is provided with a combustible feeding device does not move violently up-and-down and forms a moving bed which experiences weak fluidization. The width of the moving bed is narrow at the upper portion thereof and is spread at the lower portion due to the difference in the mass flow of the air injected from the respective air chambers. That is, the trailing end of the moving bed extends above selected air chambers and, thus, the fluidizing medium is blown upwardly by the large mass flow from these chambers so as to be displaced therefrom whereby a part of the moving bed above the remaining air chamber descends by gravity. With such downward movement of a part of the moving bed, the fluidizing medium is supplemented from the fluidized bed accompanying a circulating flow towards the upper portion of the moving bed and with the repetition of the above, as a whole, the circulating fluidized bed is moved.
Although fluidized bed combustion systems constructed in accordance with the teachings of the aforereferenced patents under actual operating conditions have, for their intended purpose, provided adequate performances to date, a need has nevertheless been evidenced for modifications to be made thereto. More specifically, a need is being evidenced in the prior art for a new and improved fluidized bed combustion system that would be applicable, in particular, for use to effect the incineration therewith of wood waste/sludge, i.e., wood waste/paper de-inking solids, mixtures that have high moisture, i.e., up to 60%, and ash contents which makes them difficult to burn utilizing prior art forms of combustion systems. Moreover, there has been evidenced in the prior art a need for such a new and improved fluidized bed combustion system that would be particularly characterized in a number of respects. To this end, one such characteristic which such a new and improved fluidized bed combustion system would desirably possess is that the high moisture content wood waste/sludge, i.e., biomass, mixtures, which commonly are non-homogenous, be capable of being metered and introduced with a high degree of reliability into such a new and improved fluidized bed combustion system. Another characteristic which such a new and improved fluidized bed combustion system would desirably possess is the capability of enabling the biomass mixtures to be dried with minimum solid particle carryover and with minimum power consumption. A third characteristic which such a new and improved fluidized bed combustion system would desirably possess is the capability to provide enhanced internal recirculation of solids resulting from the optimization of the fluid bed width/depth/height, of the arch geometry/position, of the floor slope, of the fluidizing air velocity ratio, of the fluidizing air nozzle spacing and of the bed particle size distribution. A fourth characteristic which such a new and improved fluidized bed combustion system would desirably possess is the capability to effect a covering with hot solids of the biomass mixtures upon the introduction thereof into such a new and improved fluidized bed combustion system as well as the capability to effect thereafter the lateral dispersal of the biomass mixtures through the internal recirculation of the bed solids. A fifth characteristic which such a new and improved fluidized bed combustion system would desirably possess is the capability therewith of enabling inert/tramp material to be segregated at the lowermost portion of the fluidized bed such as to permit the removal thereof by means of a non-mechanical bed cleaning system. A sixth characteristic which such a new and improved fluidized bed combustion system would desirably possess is the capability to permit heat removed from the inert/tramp material during the cooling thereof to be returned to such a new and improved fluidized bed combustion system. A seventh characteristic which such a new and improved fluidized bed combustion system would desirably possess is the capability therewith of enabling large diameter solids in addition to the inert/tramp material to be removed from such a new and improved fluidized bed combustion system by means of a non-mechanical bed cleaning system. An eighth characteristic which such a new and improved fluidized bed combustion system would desirably possess is the capability to provide therewith aggressive internal solids circulation such as to thereby reduce the chances of large agglomerations of biomass mixtures forming by virtue of agglomerations being removed from the walls in the bed/freeboard transition area, by virtue of agglomerations being broken up within the bed, and by virtue of minimizing the formation of local hot spots within the fluidized bed due to inadequate solids mixing. A ninth characteristic which such a new and improved fluidized bed combustion system would desirably possess is that the entire arrangement be compact and ideally suited to retrofitting to existing steam generators. A tenth characteristic which such a new and improved fluidized bed combustion system would desirably possess is the capability to permit therewith maintenance of bed temperature and overfire air control to be effected by means of a simple control system. An eleventh characteristic which such a new and improved fluidized bed combustion system would desirably possess is the capability to permit therewith relatively constant levels of excess air to be maintained as load is decreased by virtue of reducing the air flow to selected portions of the fluidized bed as contrasted to certain prior art forms of fluidized bed combustion systems wherein excess air must be increased and overfire air must be decreased as load is decreased in order to avoid slumping the fluidized bed. To thus summarize, a need has thus been evidenced in the prior art for such a new and improved fluidized bed combustion system that would be especially applicable for use to effect the incineration therewith of waste material, and in particular wood waste/sludge, i.e., the wood waste/paper de-inking solids generated as a by-product of the paper recycling and de-inking operations that are conducted principally in the paper and pulp industry.
It is, therefore, an object of the present invention to provide a new and improved combustion system suitable for use to effect therewith the incineration of waste materials in particular.
It is another object of the present invention to provide such a new and improved combustion system for incinerating waste materials which is characterized in that it is of the fluidized bed type.
It is a further object of the present invention to provide such a new and improved fluidized bed combustion system that is particularly suited for use to effect therewith the incineration of waste material when such waste material comprises biomass material.
Another object of the present invention is to provide such a new and improved fluidized bed combustion system that is particularly suited for use to effect therewith the incineration of biomass material when such biomass material comprises wood waste/paper de-inking solids that have been generated as a by-product of paper recycling and de-inking operations of the type that are conducted by the paper and pulp industry.
A still other object of the present invention is to provide such a new and improved fluidized bed combustion system for incinerating such wood waste/paper de-inking solids which is characterized in that the wood waste/paper de-inking solids are subjected to drying prior to being incinerated.
A further object of the present invention is to provide such a new and improved fluidized bed combustion system for incinerating wood waste/paper de-inking solids which is characterized in that the drying of the wood waste/paper de-inking solids is accomplished by effecting the covering thereof with hot solids as the wood waste/paper de-inking solids are being introduced into the fluidized bed combustion system.
A still further object of the present invention is to provide such a new and improved fluidized bed combustion system for incinerating wood waste/paper de-inking solids which is characterized in that any inert/tramp materials as well as large diameter solids entrained with the wood waste/paper de-inking solids are capable of being segregated therefrom and thereafter removed from the fluidized bed combustion system.
Yet another object of the present invention is to provide such a new and improved fluidized bed combustion system for incinerating wood waste/paper de-inking solids which is characterized in that heat is capable of being both removed from such inert/tramp material as well as large diameter solids during the cooling thereof and of then being recycled to the fluidized bed combustion system.
Yet a further object of the present invention is to provide such a new and improved fluidized bed combustion system for incinerating wood waste/paper de-inking solids wherein such incineration thereof is accomplished in an environmentally effective and efficient manner.
Yet another object of the present invention is to provide such a new and improved fluidized bed combustion system for incinerating wood waste/paper de-inking solids which is characterized by being equally well suited for use in retrofit applications as well as new applications.
Yet still another object of the present invention is to provide such a new and improved fluidized bed combustion system for incinerating wood waste/paper de-inking solids which is characterized by being easy to employ, by being reliable in operation, but which yet is relatively inexpensive to provide.